Light emitting diodes (LEDs) can convert electrical energy into optical energy for lighting and optical signaling. In general, LEDs are semiconductor diodes, typically containing a p-i-n junction. When an LED is forward biased, a current of electrons from the n-type material of the diode and holes from the p-type material of the diode combine. LEDs generally employ materials that create a suitable energy difference between the conduction band of electrons and the valence band of holes, so that the combination of an electron and a hole can spontaneously emit a photon. The energy difference is generally limited by the available materials but can otherwise be tuned or chosen to produce a desired frequency of light. Additionally, an LED can employ multiple layers of materials with conduction bands of different energies to create a quantum well that tends to confine electrons or holes and enhance the rate of spontaneous emissions, thereby improving energy efficiency of light production.
The spontaneous emission rate of a quantum well in an LED is not an intrinsic property of the quantum well, but instead depends on the electromagnetic environment of the quantum well. A plasmonic LED can exploit this phenomenon by positioning a quantum well close to a metal that supports the formation of surface plasmon polariton with electron-plasma oscillations extending into the quantum well. These electron-plasma oscillations or plasmons increase the electron-hole pair recombination rate within the quantum well via the Purcell effect and decrease the delay between a change in the current driving the LED and the corresponding change in the light emitted from the LED. Plasmonic LEDs can emit light with a modulation speed of about 10 GHz or faster while maintaining a radiative efficiency above about 20%, which compares well with the modulation speeds and efficiencies of VCSELs and other semiconductor lasers. International App. No. US/2008/001319, entitled “PLASMON ENHANCED LIGHT-EMITTING DIODES” describes some prior plasmonic LEDs that are fast enough for use in high data rate signaling.
One concern in manufacture of plasmonic LEDs is the materials available that are able to support surface plasmons of the proper frequencies for a plasmonic LED. Considering the limitations on the frequency of the emitted light placed by the available materials suitable for LEDs, silver and gold have been found to have surface plasmons with a desirable coupling for improving the response of an LED. Unfortunately, silver and gold, which must be close to a quantum well to provide the desired enhancement, have a tendency to migrate or diffuse in the semiconductor materials used in LEDs, and this diffusion can cause rapid degradation and shorting of the LED.
Use of the same reference symbols in different figures indicates similar or identical items.